Wireless communication systems operate using information modulated onto a radio frequency (RF) carrier to form an RF signal that is subsequently transmitted over a channel. Often the RF signals are formed from in-phase (I) and quadrature-phase (Q) signal components. The RF signal is received by a radio receiver, and the modulated information contained in the RF signal is demodulated to recover the desired information. Demodulation strategies are well known in the art; however receiver topologies generally introduce noise or distortion into the demodulated signal. The amount of noise or distortion depends on the specifics of the receiver configuration and the type of information modulated into the signal.
Direct Conversion Receivers (DCRs) are incorporated into many communication systems as the receiver topology of choice because of their relatively low cost, small size, and operational flexibility over a wide range of channel spacing (i.e., the frequency difference between adjacent channels used to transmit data). A receiver is referred to as a DCR if the down conversion mixer is sourced by a Local Oscillator (LO) having a frequency approximately equal to the desired RF carrier frequency, thereby converting the received RF signal to a baseband signal. A DCR receiver may also include receiver systems where the frequency difference between the LO and the desired RF carrier signal is less than several hundred kiloHertz (kHz) which may also be called a Very Low Intermediate Frequency (VLIF) configuration for the DCR topology.
Generally, DCR topologies include baseband signal paths where the complex IQ signals from the down mixer output are further processed. In addition to distortion in the received RF signal due to DC offset errors and netting errors, because of the physical limitations intrinsic to the fabrication process of the integrated circuits forming the receiver, inter-stage mismatching, process variations, and intra-stage component tolerances of the circuits, amplitude and phase imbalances between the I and Q signal paths may be introduced into the received RF signal by the DCR. The amplitude and/or phase IQ imbalance may vary over time and temperature and thus be difficult to eliminate using direct IQ compensation techniques. The distortion products resulting from the IQ imbalance may consequently degrade the fidelity of the desired information that is modulated into the RF signal.
RF signals frequency modulated (FM) with analog voice audio (typically between 300 Hz to 3 kHz) or sub-audible signaling usually have a higher sensitivity to IQ imbalance effects than signals employing N-level binary FM coding techniques. This is due to the ability of the human ear to detect very low-level harmonic resonances, such as those that occur during unvoiced speech or between breaths, in an otherwise quieted signal. Because of the historical omnipresence of analog FM voice communication systems, many of the current and future communication systems will include analog FM voice capability so as to provide backward compatibility to legacy systems and to function as a “fail safe” means of interoperability between otherwise incompatible modulation strategies. Receivers that desire high fidelity audio while demodulating a received FM signal may be precluded from using DCR topologies due to the introduction of unacceptable audio distortion.
It is therefore desirable to have a scalable, distortion-free means of demodulating analog FM signals that is compatible with DCR topologies while being ostensibly immune to IQ imbalance so as to realize the advantages of the DCR configuration while mitigating its limitations.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments shown so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Other elements, such as those known to one of skill in the art, may thus be present.